Antimicrobial resistance mechanisms

By Telesphory Wamara and Sarafina Msigwa.

Antimicrobial drug resistance is the acquired ability of a microorganism to resist the effects of an antimicrobial agent to which it is normally susceptible, or  we can define a resistant organism as one that will not be inhibited or killed by an antibacterial agent at concentrations of the drug achievable in the body after normal dosage. Some species are innately resistant to some families of antibiotics because they lack a susceptible target, are impermeable to or enzymatically inactivate the antibacterial agent, the Gram-negative rods with their outer membrane layer exterior to the cell wall peptidoglycan are less permeable to large molecules than Gram-positive cells.  No single antimicrobial agent inhibits all microorganisms, and some form of antimicrobial drug resistance is an inherent property of virtually all microorganisms, Several factors are associated with emergence of resistance among organisms. These factors include Widespread, inappropriate use of broad-spectrum antibiotics, especially in daycare centers and ICUs. (e.g. treatment of viral illnesses with antibiotics, use of antibiotics in animal husbandry and fisheries to prevent infection and increase animal growth excessive use of antimicrobial preparations in soaps and cleaning solutions in non-healthcare facilities.  Increased numbers of immunocompromised patients requiring prolonged courses of antibiotics, Prolonged survival of debilitated patients.  International travel promoting the movement of resistant bacteria (e.g.Mycobacterium tuberculosis), poverty leading to inadequate antibiotic usage because of the increasing expense of adequate antimicrobial therapy.

This article aims at highlighting the mechanisms on  how different micro organism(bacteria) develop resistance to different anti bacteria agents(antibiotics)

  For any of at least six different reasons, some microorganisms are naturally resistant to certain antibiotics

1.The organism may lack the structure an antibiotic inhibits. For instance, some bacteria, such as the mycoplasmas, lack a bacterial cell wall and are therefore naturally resistant to penicillins.

2.The organism may be impermeable to the antibiotic. For example, most gram-negative Bacteria are impermeable to penicillin G.

3.The organism may be able to alter the antibiotic to an inactive form. Many staphylococci contain β-lactamases, an enzyme that cleaves the β-lactam ring of most penicillins

4.The organism may modify the target of the antibiotic. In the laboratory, for example, antibiotic-resistant cells can be isolated from cultures that were grown from strains uniformly susceptible to the selecting antibiotic. The resistance of these isolates is usually due to mutations in chromosomal genes. In most cases, antibiotic resistance mediated by chromosomal genes arises because of a modification of the target of antibiotic activity (for example, a ribosome)

 5.The organism may develop a resistant biochemical pathway. For example, many pathogens develop resistance to sulfa drugs that inhibit the production of folic acid in Bacteria . Resistant bacteria modify their metabolism to take up preformed folic acid from the environment, avoiding the need for the pathway blocked by the sulfa drugs.

6.The organism may be able to pump out an antibiotic entering the cell, a process called efflux.

The following are the specific resistance mechanism for different antibacterial classes discussed in detail

 

Beta lactams

Resistance to beta-lactams may involve one or more of the three possible mechanisms

Resistance by alteration in target site

Methicillin-resistant staphylococci (e.g. Staph. aureus, Staph. epidermidis – MRSA, MRSE,  respectively) synthesize an additional PBP (PBP2a) which has a much lower affinity for betalactams than the normal PBPs and is therefore able to continue cell wall synthesis when the other PBPs are  inhibited. Although the mecA gene which codes for PBP2a is present on the chromosome in all cells of a resistant population, in many instances it may only be transcribed in a proportion of the cells, resulting in a phenomenon known as ‘heterogeneous resistance’. In the laboratory, special cultural conditions are used to enhance expression and demonstrate resistance. Methicillin-resistant staphylococci commonly produce beta-lactamase  and are resistant to all other beta-lactams with the exception of ceftaroline, the first cephalosporin approved by the US FDA for activity against MRSA. This cephalosporin binds to PBP2a with an affinity 2000-fold better than other beta-lactams, and is thus effective in treating infections caused by MRSA. Other organisms such as Streptococcus pneumoniae, Neisseria gonorrhoeae and Haemophilus influenzae may also utilize PBP changes to achieve beta-lactam resistance, which may vary depending on the compound employed

 

Resistance by alteration in access to the target site

 This mechanism is found in Gram-negative cells where betalactams gain access to their target PBPs by diffusion through protein channels (porins) in the outer membrane. Mutations in porin genes result in a decrease in permeability of the outer membrane and hence resistance. Strains resistant by this mechanism may exhibit cross-resistance to unrelated antibiotics that use the same porins.

Resistance by production of beta-lactamases

 Beta-lactamases are enzymes that catalyze the hydrolysis of the beta-lactam ring to yield microbiologically inactive products. Genes encoding these enzymes are widespread in the bacterial kingdom and are found on the chromosome and on plasmids ,to date, hundreds of different beta-lactamase enzymes have been described. All have the same function but with differing amino acid sequences that influence their affinity for different beta-lactam substrates. Some enzymes specifically target penicillins or cephalosporins, while others are especially troublesome in broadly attacking most beta-lactam compounds (i.e. extended-spectrum beta-lactamases, ESBLs). Some beta- lactam antibiotics (e.g. carbapenems) are hydrolyzed by very few enzymes (beta-lactamase stable), whereas others (e.g. ampicillin) are much more labile. Beta-lactamase inhibitors such as clavulanic acid are molecules that contain a beta-lactam ring and act as ‘suicide inhibitors’, binding to beta-lactamases and preventing them from destroying beta lactams. They have little bactericidal activity of their own.

Glycopeptides

Some organisms are intrinsically resistant to glycopeptides

 Gram-negative bacteria are ‘naturally’ resistant to the glycopeptides, since these compounds are too large to efficiently move through the outer membrane to the peptidoglycan. Other organisms have an altered glycopeptide target, such as pentapeptides, terminating in D- alanine-D-lactate (e.g. Erysiplothrix, Leuconostoc, Lactobacillus and Pediococcus) or D-alanine-D-serine (e.g. Enterococcus  gallinarum, Enterococcus casseliflavus)

Organisms may acquire resistance to glycopeptides

 Historically, the most clinically relevant acquired glycopeptide resistance has been observed in Enterococcus faecium and Enterococcus faecalis (vancomycin-resistant enterococci; VRE), first reported by investigators in the UK in 1986. Since that time, a variety of resistance phenotypes have been described which can be differentiated by transferability (e.g. plasmid association), inducibility and extent of resistance . The genes associated with the highest levels of glycopeptide resistance are vanA, vanB, and vanD which encode a ligase producing pentapeptides terminating in D-alanine-D-lactate . VanA is the best understood mechanism of acquired glycopeptide resistance The vanA gene is carried on a plasmid and encodes an inducible protein that is involved in cell wall synthesis in E. Coli. These proteins are responsible for synthesizing peptidoglycan precursors that have a different amino acid sequence from the normal cell wall peptidoglycan. This newly modified peptidoglycan binds glycopeptide antibiotics with reduced affinity, thus leading to resistance to vancomycin and teicoplanin. 

Aminoglycosides

Production of aminoglycoside-modifying enzymes is the principal cause of resistance to  aminoglycosides. Although relatively uncommon, resistance to aminoglycoside antibiotics may occur by alteration of the 30 S ribosomal target protein (e.g. a single amino acid change in the P12 protein prevents streptomycin binding). Resistance may also arise through alterations in cell wall permeability or in the energy-dependent transport across the cytoplasmic membrane. Production of aminoglycoside-modifying enzymes is the most important mechanism of acquired resistance . The genes for these enzymes are often plasmid mediated, located on transposons, and transferable from one bacterial species to another. The enzymes alter the structure of the aminoglycoside molecule, thus inactivating the drug. The type of enzyme determines the spectrum of resistance of the organism containing it 

Tetracyclines

The primary mechanism for decreased accumulation of tetracycline is due mainly to active efflux of the antibiotic across the cell membrane. Decreased uptake of tetracycline from outside the cell also accounts for decreased accumulation of tetracycline inside resistant cells. Tetracycline resistance genes are generally inducible by subtherapeutic concentrations of tetracycline which emphasizes the importance of adequate dosing. Pseudomonas aeruginosa and Staphylococcus aureus are bugs that display this type of resistance to tetracycline. This system may also represent a potential mechanism of resistance to the newer quinolones, but has not been found to be common among quinolone-resistant clinical isolates

 Chloramphenicol

The most common mechanism of chloramphenicol resistance involves the inactivation of the drug by a plasmid- mediated enzymatic mechanism which is easily transferred within Gram-negative bacterial populations. Chloramphenicol acetyl transferases produced by resistant bacteria  are intracellular, but are capable of inactivating all chloramphenicol in the immediate environment of the cell. Acetylated chloramphenicol fails to bind to the ribosomal target

 Macrolides, lincosamides and streptogramins

These three groups of antibacterial agents share overlapping binding sites on ribosomes, and resistance to macrolides confers resistance to the other two groups. The clinically important drugs are the macrolide erythromycin, the lincosamide clindamycin, and the streptogramin combination quinupristin-dalfopristin .Resistance is primarily due to either plasmid-encoded mef or erm genes, for efflux or alteration in the 23 S rRNA target by methylation of two adenine nucleotides in the RNA, respectively. The methylase enzyme may be either inducible or constitutively expressed. Erythromycin is a better inducer of resistance than the lincosamides, but strains resistant to erythromycin will also be resistant to lincomycin and clindamycin, so-called ‘MLS (macrolide-lincosamide- streptogramin) resistance’. Induction also varies between bacterial species, and resistant strains of Gram-positive cocci such as staphylococci and streptococci are common. In contrast to methylation, efflux is only active against macrolide drugs and does not confer lincosamide and streptogramin resistance.

Oxazolidinones

Oxazolidinones are a new class of synthetic bacteriostatic antimicrobial agents . Linezolid, the oxazolidinone currently available, is active against a wide range of Gram-positive bacteria, including multiresistant strains. Linezolid inhibits initiation of protein synthesis  by targeting 23S ribosomal RNA in the 50S subunit in a manner which prevents formation of a functional 70 S complex. Due to the drug's unique mechanism of action, resistance mutations (i.e. altered target) are rare and seen primarily in Enterococcus faecium

Quinolones

Quinolones are synthetic agents that interfere with replication of the bacterial chromosome

Resistance to quinolones is usually chromosomally mediated 

Chromosomally mediated resistance is exhibited in two forms:

• Mutations, which change the target enzymes in a manner that affects quinolone binding

• Changes in cell wall permeability, resulting in decreased uptake, or by efflux. These mechanisms may also lead to cross-resistance to other unrelated agents affected by the same process. Plasmid-encoded quinolone resistance involves production of a protein (termed qnr) that protects the target DNA from quinolone binding. This protein has been shown to act in concert with a plasmid encoded enzyme capable of reducing the activity of some fluoroquinolones, resulting in increased levels of quinolone resistance

Rifamycins

Rifampicin is clinically the most important rifamycin and blocks the synthesis of mRNA,

The primary use for rifampicin is in the treatment of mycobacterial infections, but resistance is a concern, resistance is provided by chromosomal mutations that alter the RNA polymerase target, which then has lowered affinity for rifampicin and escapes inhibition. The prevalence of rifampicin-resistant M. tuberculosis is increasing, threatening the future of its use in antituberculosis therapy

Trimethoprim

Resistance to trimethoprim is provided by plasmid encoded dihydrofolate reductases. Plasmid-encoded dihydrofolate reductases with altered affinity for trimethoprim allow the synthesis of THFA to proceed unhindered by the presence of trimethoprim. The ‘replacement enzymes’ are approximately 20 000-fold less susceptible to trimethoprim while retaining their affinity for the normal substrate. Bacteria that are resistant to sulphonamide and trimethoprim are also resistant to co-trimoxazole

Nitroimidazoles

Metronidazole is a nitroimidazole with anti parasitic and antibacterial properties

Metronidazole resistance is of increasing concern in T. vaginalis, G. intestinalis, and several anaerobic and microaerophilic bacteria, and commonly involves either an alteration in uptake or a decrease in cellular reductase activity, thereby slowing the activation of the intracellular drug. Helicobacter pylori, a microaerophilic bacterium causing ulcers and gastritis, has been frequently treated with metronidazole. However, resistance can rapidly develop

DECREASING ANTIMICROBIAL RESISTANCE

In order to minimize antibiotic resistance in your patients you must employ these resistance management approaches:

• Withhold antibiotics in situations where they are not likely to benefit the patient for self-limited          viral infections such as "the common cold". Symptomatic treatment and supportive measures are the   most appropriate care and antibacterial agents are not indicated.
• Use the narrowest spectrum antimicrobial agent possible to treat an infection. For example, a semisynthetic penicillin or even oral penicillin would be a much better choice for treatment of a staphylococcal infection than a broad spectrum fluoroquinolone or cephalosporin. This works well provided the organism is known or likely to be susceptible to the narrower spectrum antibiotic
•    Base decisions about broadness of empiric antibiotic coverage on the severity of illness.   For  example, in the case of a patient who is clinically stable and not at risk for significant      morbidity if a resistant pathogen is not treated immediately, it may be appropriate to  begin a    narrow spectrum agent while awaiting culture and susceptibility data.
•  Emphasize prevention of infection through careful hygiene, especially handwashing and other measures to control the spread of pathogens. It sounds really simple, but proper and adequate handwashing by healthcare professionals can prevent many cases of infection due to virulent and antibiotic-resistant pathogens
•  Utilize education to achieve therapeutic and preventative goals. Patients and families should be counseled as to when antibiotics are needed, how to take them correctly and for the proper duration. Education can also be used to foster earlier detection of therapeutic failure, which may be critical when treating patients who may be infected with antibiotic-resistant pathogens. Our communities must be cautioned against buying cleaning products with antimicrobial properties as well as using feed lot antibiotics